Problem: The scale on a map is 10cm : 4km. If the distance between two cities is 32km, how far apart in cm are the two cities on the map?
Explanation: The scale means that every 10cm on the map corresponds to an actual distance of 4km. An actual distance of 32km is the same as 8 $\cdot$ 4km. The distance between the two cities on the map is 8 $\cdot$ 10cm, or 80cm.